1. Field of the Invention
The present application relates generally to atomic layer deposition of conductive titanium oxide thin films. The conductive titanium oxide films may serve, for example, as an interface layer between a high k material and a metal electrode.
2. Background
Transparent conducting oxides are used in a wide variety of applications, including flat panel displays, light emitting diodes and solar cells. Sn-doped In2O3 (ITO) has been commonly used but suffers from some drawbacks. Recently, it has been found that the conductivity of TiO2 can be increased by doping with niobium or tantalum. These doped anatase thin films have a resistivity value comparable to conventional ITO, making TiO2:Nb and TiO2:Ta candidates for a new transparent conducting oxide.
ALD is a self-limiting process, whereby alternated pulses of reaction precursors saturate a substrate surface and leave no more than one monolayer of material per pulse. The deposition conditions and precursors are selected to ensure self-saturating reactions, such that an adsorbed layer in one pulse leaves a surface termination that is non-reactive with the gas phase reactants of the same pulse. A subsequent pulse of different reactants reacts with the previous termination to enable continued deposition. Thus, each cycle of alternated pulses leaves typically less or no more than about one molecular layer of the desired material. The principles of ALD type processes have been presented by T. Suntola, e.g. in the Handbook of Crystal Growth 3, Thin Films and Epitaxy, Part B: Growth Mechanisms and Dynamics, Chapter 14, Atomic Layer Epitaxy, pp. 601-663, Elsevier Science B.V. 1994, the disclosure of which is incorporated herein by reference. Variations of ALD have been proposed that allow for modulation of the growth rate. However, to provide for high conformality and thickness uniformity, these reactions are still more or less self-saturating.
Atomic layer deposition of conducting oxides has focused mainly on SnO2, ITO and ZnO. These materials contain elements that are uncommon in integrated circuits (ICs) and zinc in particular is not used in the IC context. ALD of mixed titanium oxide films has thus far been concerned with improving the insulating properties of the films, not with preparing conductive films.